The present invention relates to monitoring non-linear distortion in the transmission of voice and voice band data. More particularly, the present invention relates to a non-linear distortion meter and circuitry employed therein.
Common carrier telephone channels, both switched and dedicated, now carry a variety of non-voice signals. These signals usually consist of one or more tones which are amplitude and/or phase modulated by either analog or digital information. Each signal format is impaired to varying degrees by the physical limitations, interferences, and design compromises imposed by nature and economics. Naturally it is desirable to be able to quickly and simply measure the limitations of a transmission channel; however, until the present invention there has been no really practical approach to measuring the full effects of amplitude distortion in a telephone channel.
Amplitude distortion on voice frequency telephone channels can degrade voice quality and seriously impair data transmission. Unfortunately, conventional distortion measuring techniques are inadequate for telephone channel tests. A simple harmonic distortion test has a serious measurement uncertainty in channels having multiple distortion sources combined with envelope delay or frequency offset. Total distortion tests, on the other hand, have difficulty distinguishing distortion from channel noise; further, such tests cannot separate the various types of distortion sufficiently to assist in fault isolation.
Bell System Technical Reference, PUB 41008, entitled "Analog Parameters Affecting Voiceband Data Transmission -- Description of Parameters", and dated October 1971, describes an intermodulation technique for measuring non-linear distortion in voice channels. Pages 16-24 of this publication presents a detailed analysis of the common sources of both second and third order distortion products and the effects of typical channel conditions on their measurement. The publication further illustrates why the intermodulation technique is more accurate and useful than the harmonic and total distortion techniques.
The intermodulation technique recommended by the aforementioned Bell System Technical Reference utilizes as test signals two narrow bands of Gaussian noise, centered about 860 Hz and 1380 Hz, respectively. The noise bands are used rather than discrete tones because the noise spectrum produced by individual tones in PCM systems is not flat and continuous but instead is discrete; the discrete components add or beat with the non-linear distortion product being measured causing inaccurate and timevariable readings. The noise bands produce a flat continuous spectrum. Moreover, the crest factor (i.e. -- ratio of peak value to RMS value) is on the order of 10 db for a Gaussian distribution, thereby assuring that test signal peaks will be large enough to test the region of channel non-linearity; if two individual test tones are employed the crest factor is only 6 db and a thorough test of the channel non-linearity is not assured.
The noise bands are applied to the channel under test in which second and third order intermodulation distortion is to be measured. Third order distortion is measured as the signal component produced by the channel at 1900 Hz (i.e. 2 .times. 1380 - 860). Second order distortion is measured as the signal components produced by the channel at 520 Hz (i.e. 1380 - 860) and 2240 Hz (i.e. 860 + 1380).
The intermodulation measurement technique described above is fine in theory. In practice, however, the procedure is tedious because it is necessary to wait for at least 30 seconds, and usually more, for each reading to stabilize. This is due to the random nature of the noise band test signals. Specifically, within each narrow noise band the signal amplitude is changing randomly and arbitrarily small difference frequencies exist. Theoretically, the signal detector should average out these small difference frequencies and to do so would require an infinite time. In practice, a relatively long time period, on the order of a minute, is required in order to obtain a meaningful result.
It is therefore an object of the present invention to provide an intermodulation measurement technique which is as meaningful and accurate as that described above but which can be performed in a matter of a few seconds.
It is another object of the present invention to provide an improved intermodulation measurement technique which eliminates the need for using noise bands as test signals.
It is still another object of the present invention to provide a practical non-linear distortion analyzer and test set for telephone channels.
It is still another object of the present invention to provide novel circuits for particular use in a distortion analyzer and test set, including an automatic gain control, an RMS converter, and a non-linear circuit for producing stable and predictable second and third order distortion.